Circuit for operating an electric motor during shut down

ABSTRACT

A circuit for operating an electric motor (10) has a triac (24) that is parallel to the electric motor (10). The triac (24) is acted upon by a first control signal (28) that is dependent on the voltage present at a first connection (11) of the electric motor (10). The triac (24) is further acted on by a second control signal (22) that is emitted as a braking signal by a motor-control circuit (19). The circuit of the invention, which includes a semiconductor component (24), permits both a braking of the electric motor (10) and the damping of an inductive voltage peak during the switch-off process of the electric motor (10). The damping of the inductive voltage peak is also assured after the separation of the voltage from an energy source (16).

RELATED ART

The invention is based on a circuit for operating an electric motor ofthe generic type having a semiconductor component contained in a circuitin parallel with the motor.

U.S. Pat. No. 3,487,282 discloses a generic circuit having a thyristorthat is contained in a circuit in parallel to a motor. The thyristor isprovided for short-circuiting the motor to permit fast braking. Aswitching transistor that connects a connection of the electric motor toa voltage source as a function of a control signal is provided forswitching the electric motor on and off. The inductive voltage peakappearing at the electric motor during the connected-off process iseliminated by a diode switched in parallel to the electric motor.

The abstract of JP 1043082 discloses a further circuit that has abidirectional thyristor (triac) switched in parallel to the motor. Thisthyristor is switched on due to a stop signal, causes a short-circuitcurrent and brakes the motor by means of a short-circuit brake.

It is the object of the invention to provide a circuit for operating anelectric motor which, with simple means, permits the damping ofinductive voltage peaks and braking of the electric motor.

SUMMARY AND ADVANTAGES OF THE INVENTION

The above object generally is achieved according to the presentinvention by a circuit for operating an electric motor having a triacthat is contained in a circuit in parallel to the electric motor, and acontrol signal that is produced by a signal generator for the triac,with the control signal enabling the current path in one direction inthe triac. A further control signal that is dependent on the voltageappearing at a connection of the electric motor is provided for thetriac and enables the current path in the opposite direction in thetriac.

The circuit of the invention can be embodied particularly inexpensively.In accordance with the invention, a bidirectional thyristor (triac) isprovided that is connected in parallel to the electric motor. The triacincludes two thyristors that operate in anti-parallel and are integratedinto a single silicon crystal. The component includes only one controlelectrode, by way of which the current path can be enabled (triggered)in the two directions by a pulse, depending on the polarity of theexternal voltage. The bidirectional thyristor is referred to hereinafteras a triac. The triac takes over both the damping of the inductivevoltage peak during the switch-off process of the electric motor and theshort-circuiting of the electric motor during the braking process. Toembody the circuit of the invention, only a single semiconductorcomponent is required that is also available in a configuration of asurface-mounted device (SMD). The SMD embodiment requires little spaceon a printed circuit board, and can be attached automatically, resultingin cost advantages in mass production.

The triac provided in accordance with the invention is acted upon by afirst control signal that is dependent on the voltage appearing at theelectric motor. The first control signal is therefore suited for dampingthe inductive voltage peak during the switch-off process of the electricmotor. The triac is acted upon by a second control signal produced by asignal generator. The second control signal is suited for braking theelectric motor.

The basic features of the invention particularly permit reliableoperation of the electric motor in applications providing a separationof the circuit from a voltage source by means of a switch. Inparticular, the first control signal, being a function of the voltageappearing at the electric motor, ensures, in connection with the triac,the damping of the inductive voltage peak during the switch-off processof the electric motor, even after the switch has been opened.

A measure that can be implemented particularly simply provides that thefirst control signal, being dependent on the voltage appearing at theelectric motor, is obtained from the motor voltage by means of a diode.The diode connected directly to a connection of the electric motor andleading to the control input of the triac becomes conductive during theoccurrence of the inductive voltage peak.

Another advantageous measure provides that the second control signal forthe triac is provided by a motor-control circuit that emits the secondcontrol signal as a braking signal for the electric motor. Themotor-control circuit simultaneously generates switching signals for aswitching transistor that can be provided for switching the electricmotor on and off.

Further advantageous measures relate to the use of resistors by way ofwhich the two control signals for the triac are guided. The resistorspermit a limitation of the triac gate trigger current.

Further advantageous modifications and embodiments ensue from thefollowing description.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a circuit diagram of an embodiment of a circuitaccording to the invention for operating an electric motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The FIGURE shows an electric motor 10 having a first and a secondconnection 11, 12. The first connection 11 can be connected by way of aswitching transistor 13 and a switch 14 to a first current-supply line15. The first current-supply line 15 leads to an energy source 16, whichprovides an operating voltage that appears between the firstcurrent-supply line 15 and a second current-supply line 17. The secondcurrent-supply line 17 is referred to hereinafter as ground 17. Thesecond connection 12 of the electric motor 10 is connected to ground 17.

The switching transistor 13 is actuated by a switching signal 18 madeavailable by a motor-control circuit 19. The motor-control circuit 19 issupplied with energy by way of an energy-supply line 20 leading to theswitch 14, and by way of a line leading to ground 17. The motor-controlcircuit 19 includes a braking circuit 21, which emits a braking signal22 referred to hereinafter as the second control signal 22. Both theswitching signal 18 and the second control signal 22 are established bythe motor-control circuit 19 as a function of an operation-mode signal23, which is supplied to the motor-control circuit 19 from the outside.

A triac 24 is connected in parallel with the electric motor 10. Itscontrol input 25 is connected to the first motor connection 11 by way ofa first resistor 26 and a diode 27 connected in series with the firstresistor 26. A first control signal 28 appears at the diode 27. Thecontrol input 25 of the triac 24 also has applied to it, via a secondresistor 29, the braking signal referred to as the second control signal22.

The circuit of the invention for operating the electric motor 10operates as follows:

The electric motor 10 is controlled via the operation mode signal 23,which is supplied to the motor-control circuit 19. The operation-modesignal 23 further establishes, for example, whether the electric motorshould also be braked while being switched off. It is necessary to brakethe electric motor, for example, in a control drive to reduce overshootonce a predetermined position has been reached. A further applicationoption is in a windshield-wiper drive of a motor vehicle. The wiper armshould stop immediately, through sharp braking of the electric motor 10,after reaching a resting position.

When the electric motor 10 is switched off, two opposing inductivevoltages appear. An inductive voltage peak is caused by the change overtime of the current flowing through the electric motor 10. To limit thisinductive voltage peak, a free-wheeling circuit is provided parallel tothe electric motor; the energy stored in the inductance of the electricmotor 10 is discharged by way of this circuit. In the illustratedembodiment of the circuit of the invention, a negative inductive voltagepeak appears at the first connection 11 during the switch-off of theelectric motor 10, bringing the diode 27 into the conductive state. Inthis operating state, the first control signal 28 appears, and issupplied to the control input 25 of the triac 24 by way of the firstresistor 26. The free-wheeling circuit is thus closed. The first controlsignal 28 then only appears if the switch 14 is opened when the electricmotor 10 is running. Only in this instance is the free-wheeling circuit,which is otherwise closed by way of the current-supply lines 15, 17 andthe energy source 16, opened and consequently interrupted. In thisoperating state, the energy supply of the motor-control circuit 19 byway of the energy-supply line 20 is omitted at the same time, so acorresponding switch-on signal for the triac 24 can no longer beprovided. The first control signal 28, which depends on the voltage ofthe electric motor, then always occurs independently of theenergy-supply situation if the voltage at the first connection 11 of theelectric motor 10 has a value at which the diode 27 becomes conductive.

The electric motor 10 is braked with the second control signal 22generated by the motor-control circuit 19 in the braking circuit 21. Thesecond control signal 22 supplied to the control input 25 of the triac24 triggers the triac and short-circuits the electric motor 10.

The two resistors 26, 29 respectively limit the gate-trigger currentflowing in the control input 25 of the triac 24.

What is claimed is:
 1. Circuit for operating an electric motor (10),comprising a triac (24) that is contained in a circuit in parallel tothe electric motor; a signal generator (19) for producing a firstcontrol signal (22) with the signal enabling the current path in onedirection in the triac (24); and a circuit for producing a furthercontrol signal (28) for the triac (24) that is dependent on the voltageappearing at a connection point between the electric motor (10) and anenergy source, and that enables the current path in the oppositedirection in the triac (24).
 2. Circuit according to claim 1, whereinthe further control signal (28) is obtained from the first connection(11) of the electric motor (10) via a diode (27).
 3. Circuit accordingto claim 1, wherein the signal generator is a motor-control circuit thatproduces the further control signal (22) as a braking signal for theelectric motor (10).
 4. Circuit according to claim 1, wherein a firstresistor (26) is provided for limiting the current of the furthercontrol signal (28).
 5. Circuit according to claim 1, wherein a secondresistor (29) is provided for limiting the current of the first controlsignal (22).
 6. A motor control circuit comprising:an electric motorconnected via a switch across a source of energy; a triac connected inparallel with the electric motor; a control circuit for controlling saidswitch and for supplying a control signal to a control electrode of thetriac to trigger the triac to conduct in a first direction; and a diodethat is connected between the control electrode of the triac and aconnection of the motor with the switch and that has a polarity totrigger the triac to conduct in the opposite direction when the polarityof the voltage of said connection reverses from its normal polarity.